Chemistry Reference
In-Depth Information
In the case of ditopic molecules with two hydrogen bonding groups, linear
noncovalent polymers can form in either solution or the solid state (Fig. 4.1). In
this case, the degree of noncovalent polymerization (DP) depends directly on the
association constant (K
a
) in the medium and the concentration of the molecule (c)
as shown in Eq. (4.3) (Xu et al. 2004):
DP
¼
4K
a
c
=
(
1
þ
(1
þ
8K
a
c)
0
:
5
)
(2K
a
c)
0
:
5
(4
:
3)
Another feature of hydrogen bonding systems is that they are dynamic, and this
often allows them to achieve the most thermodynamically favored state. This
dynamic property of hydrogen bonding allows self-assembly to occur at
ambient conditions, unlike many covalent polymerizations (Saadeh et al. 2000).
Deans and colleagues (1999) were able to create a thermodynamic cycle to
examine the competitive processes of self-association of a hydrogen bonding
polymer and association with a small molecule complementary guest in solution.
In the case of hydrogen bonded networks, the dynamic character leads to the pre-
sence of fewer network defects, such as dangling ends and unincorporated chains
(sol). In support of this hypothesis, Lange et al. (1999) observed a higher plateau
modulus for a hydrogen bonded network compared to a covalent network.
Karikari et al. (2007) studied nucleobase-functionalized poly(
D
,
L
-lactide) star
polymers and observed higher viscosity for the solution blends of complimenta-
rily functionalized polymers.
Figure 4.1 Degree of noncovalent polymerization as a function of the association constant
(K
a
). The effect of concentration is illustrated in the two parallel
lines. Reprinted from
Brunsveld et al. (2001). Copyright 2001 American Chemical Society.
